1. Field of the Invention
The present invention relates to a method of cleaning a light exposure mask, that is a photo-mask, to be used for manufacturing of semiconductor devices such as IC's, LSI's, VLSI's, etc. (hereinafter called simply "mask") and a cleaning apparatus for practicing the method.
2. Description of the Prior Art
One of the important steps in a manufacturing process of semiconductor devices is a light exposure step. The mask to be used in this step must be free from dust or dirt. More particularly, straying dust would adhere on the front and rear surfaces of the mask because the electric charges are accumulated in the patterns on the mask by irradiating ultraviolet rays or far-ultraviolet rays in this step. In one method of cleaning the mask in the prior art, only brush-cleaning of either one or both of the front and rear surfaces of the mask was performed with the aid of a neutral detergent or pure water, and thereafter drying was effected by means of Freon. Alternatively, in another method of cleaning of the mask in the prior art, only shower-cleaning of either one or both of the front and rear surfaces of the mask was performed with the aid of a neutral detergent or pure water, and thereafter drying was effected by means of Freon. However, the former method would cause electrostatic breakdown in the pattern on the mask and also had a poor cleaning effect. The latter method was poorer in the cleaning effect than the former method. Consequently, there was a posibility that the influence of dust on the mask cleaned by the prior art method would be generated in semiconductor clips of 5% or more in a semiconductor wafer. Furthermore, in a reduced-size projection light exposure apparatus, a reticle mask used in the apparatus must be free from dust of 3 .mu.m or more in size and stains or contaminations produced upon drying of a cleaning liquid. However, a mask cleaning technique which fulfils such type of requirements and which has a good workability has not been known. On the other hand, to inspect a cleaning effect, that is, an extent of removal of dust and the like in a mask after cleaning, is an important factor for dertermining the product yield of semiconductor elements. Quality control of a mask after cleaning in the prior art, however, was directly effected in some cases by means of a microscope, and in other cases it was effected by mounting the mask to a light exposure apparatus, carrying out light exposure of a semiconductor substrate coated with a photo-resist and then observing the produced photo-resist pattern by means of a microscope. The time and labor necessitated for this observation are immeasurable.